In conventional color photography, films containing light-sensitive silver halide are employed in hand-held cameras. Upon exposure, the film carries a latent image that is only revealed after suitable processing. These elements have historically been processed by treating the camera-exposed film with at least a developing solution having a developing agent that acts to form an image in cooperation with components in the film. Developing agents commonly used are reducing agents, for example, p-aminophenols or p-phenylenediamines.
Typically, developing agents (also herein referred to as developers) present in developer solutions are brought into reactive association with exposed photographic film elements at the time of processing. Segregation of the developer and the film element has been necessary because the incorporation of developers directly into sensitized photographic elements can lead to desensitization of the silver halide emulsion and undesirable fog. Considerable effort, however, has been directed to producing effective blocked developing agents (also referred to herein as blocked developers) that might be introduced into silver halide emulsion elements without deleterious desensitization or fog effects. Accordingly, blocked developing agents have been sought that would unblock under preselected conditions of development after which such developing agents would be free to participate in image-forming (dye or silver metal forming) reactions.
U.S. Pat. No. 3,342,599 to Reeves discloses the use of Schiff-base developer precursors. Schleigh and Faul, in a Research Disclosure (129 (1975) pp. 27-30), describes the quaternary blocking of color developers and the acetamido blocking of p-phenylenediamines. (All Research Disclosures referenced herein are published by Kenneth Mason Publications, Ltd., Dudley Annex, 12a North Street, Emsworth, Hampshire P010 7DQ, ENGLAND.) Subsequently, U.S. Pat. No. 4,157,915 to Hamaoka et al. and U.S. Pat. No. 4,060,418 to Waxman and Mourning describe the preparation and use of blocked p-phenylenediamines in an image-receiving sheet for color diffusion transfer.
All of these approaches have failed in practical product applications because of one or more of the following problems: desensitization of sensitized silver halide; unacceptably slow unblocking kinetics; instability of blocked developer yielding increased fog and/or decreased Dmax after storage, lack of simple methods for releasing the blocked developer, inadequate or poor image formation, and other problems. Especially in the area of photothermographic color films, other potential problems include poor discrimination and poor dye-forming activity.
Recent developments in blocking and switching chemistry have led to blocked developing agents, including p-phenylenediamines, that perform relatively well. In particular, compounds having "p-ketoester" type blocking groups (strictly, .beta.-ketoacyl blocking groups) are described in U.S. Pat. No. 5,019,492. With the advent of the .beta.-ketoester blocking chemistry, it has become possible to incorporate p-phenylenediamine developers in film systems in a form from which they only become active when required for development. The .beta.-ketoacyl blocked developers are released from the film layers in which they are incorporated by an alkaline developing solution containing a dinucleophile, for example hydroxylamine.
In addition to the aforementioned U.S. Pat. No. 4,157,915, blocked developing agents involving .beta.-elimination reactions during unblocking have been disclosed in European Patent Application 393523 and kokais 57076453; 2131253; and 63123046, the latter specifically in the context of photothermnographic elements.
The incorporation of blocked developers in photographic elements is typically carried out using colloidal gelatin dispersions of the blocked developers. These dispersions are prepared using means well known in the art, wherein the developer precursor is dissolved in a high vapor pressure organic solvent (for example, ethyl acetate), along with, in some cases, a low vapor pressure organic solvent (such as dibutylphthalate), and then emulsified with an aqueous surfactant and gelatin solution. After emulsification, usually done with a colloid mill, the high vapor pressure organic solvent is removed by evaporation or by washing, as is well known in the art. Alternatively, solid particle (ball-milled) dispersions can be prepared using means well known in the art, typically by shaking a suspension of the material with zirconia beads and a surfactant in water until sufficiently small particle size is produced.
There remains a need for blocked photographically useful compounds with good keeping properties, which at the same time exhibit good unblocking kinetics. With respect to developing agents, it is an object to obtain a film incorporating blocked developing agents that provide good dye-forming activity and which, at the same time, yield little or no increased fog and/or provide little or no decrease in Dmax after storage.
In one application of the invention, it is a further object to obtain blocked photographically useful agents for use in photothermographic color films. With respect to developing agents, there is a continuing need for photothermographic imaging elements that contain a developing agent in a form that is stable until development yet can rapidly and easily develop the imaging element once processing has been initiated by heating the element and/or by applying a processing solution, such as a solution of a base or acid or pure water, to the element. A completely dry or apparently dry process is most desirable. The existence of such a process would allow for very rapidly processed films that can be processed simply and efficiently in photoprocessing kiosks. Such kiosks, with increased numbers and accessibility, could ultimately allow for, relatively speaking, anytime and anywhere silver-halide film development.
Similarly, there is a need for incorporating other photographically useful compounds into a photothermographic element such that they remain stable until processing and are then rapidly released. Such photographically useful compounds include, couplers, dyes and dye precursors, electron transfer agents, development inhibitors, etc., as discussed more fully below. The blocking of other photographically useful compounds, besides developing agents, are disclosed in the prior art. For example, U.S. Pat. No. 5,283,162 to Kapp et al. and U.S. Pat. No. 4,546,073 to Bergthaller disclose blocked development inhibitors, and U.S. Pat. No. 4,248,962 to Lau discloses blocked couplers wherein the blocking group in turn comprises a photographically useful group.